Nickel-base cast alloys have been used extensively to turbine parts and component designs requiring high temperature strength and corrosion resistance, for service at temperatures in excess of 1400.degree. F. Single crystal casting techniques have been developed for the manufacture of gas turbine blades and vanes and permit alloy compositions to be further optimized for high temperature strength by removal of grain strengthening elements such as carbon, boron and zirconium, which adversely affect heat treatability. The primary strengthening mechanism of nickel base super alloys cast in single crystal components, is the formation of the intermetallic gamma prime phase, Ni.sub.3 (Al, Ti). The stress-rupture life of such single crystal components, is a function of the volume fraction of fine gamma prime particles in the microstructure. Removal of grain boundary strengthening elements such as B, Zr, Hf, and C (which are also melting point depressants) resulted in an increase in the incipient melting temperatures from about 2250.degree. F. to about 2400.degree. F.
There is generally a 200.degree.-300.degree. F. difference in the temperatures to which turbine blades and stationary turbine vanes are subjected. While turbine blades are under higher stress, the stationary vanes are subjected to higher temperatures. Accordingly, it is desirable to develop a vane or turbine nozzle alloy which is more temperature resistant, i.e. has a higher incipient melting point, with less demanding high temperature strength properties.
Since the turbine vanes are subjected to higher temperatures, at which oxidation is accelerated, the oxidation resistance of turbine vane alloys must be improved, relative to blade materials.
It is therefore an object of our present invention to provide a single crystal nickel-base alloy article having a combination of improved incipient melting temperature and oxidation resistance. This and other objects are realized by our present invention as will be appreciated from the following description of the invention.